Stress plates have been used previously, for example, to attach membranes to substrates such as a roof. Such stress plates are normally provided with a plurality of spaced apart prongs that are hingedly coupled to the stress plate and extend downwardly from the bottom of the stress plate. The prongs can have a length such that they merely grip the membrane or a length sufficient to extend through the membrane and into the roof.
The membrane is placed over the substrate in order to, for example, provide sealing and water proofing of the roof. The stress plate is placed over the membrane. A fastening device, such as a screw is inserted through an opening in the center of the stress plate. The fastener device extends through the membrane and into the roof. The fastener device is thus attached to the roof causing the membrane to be secured to the roof by the stress plate and the fastener device.
When using a stress plate, sometimes known as a membrane plate or a clamping plate, for securing a membrane to a roof, insulating material may be disposed between the membrane and the roof. The insulating material can be in the form of an insulation board or other insulation material.
It is well known for a stress plate to be formed with a circular shape. For example, U.S. Pat. No. 4,787,188 (Murphy) teaches a circular stress plate. The stress plate taught by Murphy has an outer circular rib and an inner circular rib as well as a plurality of hinged prongs. The hinged prongs are disposed at equal radial distances from the center of the stress plate and circumferentially spaced apart from each other at angles of 90.degree.. Each of the hinges of the prongs taught by Murphy is aligned at an angle of 90.degree. with respect to a radius of the circular stress plate.
U. S. Pat. No. 4,282,050 (Thiis-Evensen) teaches a stress plate for cladding a roof on a support structure. The fastening process taught includes applying an insulation layer on the support structure and applying edge abutting webs of cladding material over the insulation layer. The insulation layer and the cladding layers are simultaneously mechanically anchored to the support structure using fasteners. The edges of the webs and the fasteners are sealed by welding strips. The fastener taught by Thiis-Evensen for fastening the layers to the support structure is rectangular in shape with gripping claws disposed at each end of the plate.
U.S. Pat. No. 4,543,763 (Ernst) teaches a fastening plate having circumferentially spaced apart projections disposed on the plate. The plates taught by Ernst can be round or square and are adapted to control the rate of axial penetration of an anchor in a masonry structure.
When a stress plate is attached to a membrane and a substrate to form a stress plate assembly in this manner different forces are applied to the assembly during normal use. One significant force applied to the stress plate assembly is shear force. A shear force is a force that is applied to the stress plate assembly parallel to the surface of the membrane. Shear forces can result from uplift pressure due to wind hitting the roof. In the region where the stress plate attaches the membrane to the substrate the shear forces can result in rubbing of the membrane against the stress plate and, in particular, rubbing of the membrane against the prongs of the stress plate. This rubbing can promote tearing of the membrane.